Without external help, the trailer should participate as synchronous as possible (or a little stronger) in the braking of the vehicle in front. This is especially important if the brake is only hit slightly. The system should be cheap, and easy to repair.
How it works
In earlier times, it was necessary to brake with a little more pedal force if the vehicle was coupled to a trailer. If the trailer was relatively heavy in comparison to the vehicle, the trailer was able to put the vehicle into disbalance while descending.
Modern overrun brakes are more sensitive. If the cables, pivots, and the sliding pieces are working properly, and the braking shoes are lubricated, one almost does not feel the working of the overrun brake. The force created by the overruning of the trailer while braking, is transferred via a linkage into a tractive force, and then via cable passed on to the spreading lever.